1. Field of the Invention
The present invention relates to an extruder, especially for the foam extrusion of plastics, also for the incorporation of additives and aerating agents, preferably with the supply of relatively high-viscosity or low-viscosity polymers via a lateral-arm extruder, and also cooling to unmoulding temperature and supply into a moulding tool, with a pump being connected between at least two stages or zones.
2. Background Information
The extrusion of foamed plastics has gained increasing importance. Accordingly an extensive development is recorded. Various structural shapes for extruders have arisen. Belonging thereto are single-screw extruders, twin-screw extruders and also planetary extruders. The starting material basis for extrusion has become more diverse. Apart from the starting material, additives and aerating agents are also of importance. In the field of aerating agents the specialist world first turned to fluorocarbons. These aerating agents can also result in good foaming results under difficult general conditions. Unfortunately these aerating agents had to be abandoned again, because a very great environmental threat was posed by them. Alternatively available aerating agents are all less desirable solutions.
Extruded materials produced by an extruder of this type can take many forms. For example molded plastics such as household or automotive moldings and trim can be constructed. Additionally, materials such as plastic lumber can be formed from the extrusion process. Styrofoam and other foamed styrene type materials can also be produced by such an extrusion process. The extrusion process can also be used to make intermediate plastics, which can then be transferred to subsequent processing operations to make finished products.
In view of the above background the separation of the extrusion operation into different zones and stages achieves ever greater importance, because in the individual zones an influence can be specifically exerted on the charged material. The distinction of the following zones has been known for a long time:
feeding/filling PA1 conveying/compression PA1 degassing PA1 incorporation of liquid and solid additives and also of other polymers PA1 incorporation of aerating agents PA1 homogenisation PA1 plasticising PA1 cooling PA1 discharge PA1 Choosing a smaller number of planetary spindles, PA1 choosing a different arrangement of planetary spindles on the reference diameter of the internal toothing, i.e. displacement to produce a greater distance between the spindles of individual pairs of spindles, PA1 choosing different toothing PA1 choosing a different spindle pitch and even an opposed arrangement.
These zones can be found together or partially on a single extruder. In other words, the extrusion process can be designed such that all of the zones or some combination of the zones can occur on a single extruder. Alternatively, multiple extruders can be used in such a manner that the individual zones can each occur on a different extruder, or that a single zone can occur on a series of several extruders. However the zones may also be divided over several extruders. Then multi-stage extrusion plants are frequently spoken of. A typical form of multi-stage extrusion is the so-called extrusion in cascades. In this case two extruders, for example, work together in such a manner that the one extruder charges its melt into the other extruder. In other words, one of the extruders can supply the product from that extruder to another extruder for further processing.
The above multi-stage term is to be distinguished from a multi-stage production process. The latter is mentioned, in particular for the manufacture of cross-linked plastics, and also plastic foams.
The longer the screw, the more work consumed by friction is to be overcome. In other words, a longer screw causes a greater frictional energy loss and that energy loss must then be overcome. To overcome the friction, more and more energy has to be supplied. At the same time limiting temperatures may not be exceeded. This compels an intensive cooling of different extruder zones. Apart from structural and operational expenditure caused thereby, this results in an extreme energy expenditure, which is again destroyed in the course of cooling.